because of its appearance and the taste andodors sometimes associated with it.Turbidity is a muddy or unclear conditionof water caused by particles of sand, clay, ororganic matter being held in suspension. Clayand silt remain suspended in water for thelongest period of time because of their particlesize and specific gravities. The removal ofturbidity is essential to the production ofpotable water. Removal reduces watercontamination, extends the time betweenbackwashing of filters, decreases chlorinedemand, improves disinfection, and enhancesuser acceptability of the finished water. Properwater treatment requires turbidity removalbecause suspended particles often containorganisms that may cause diseases.Turbidity is removed by coagulation andsedimentation. Since the physical characteristicsof raw water vary widely in differentlocations, dosages of coagulant chemicals mustbe determined at each water point to ensuremaximum efficiency with minimal waste ofchemicals. After coagulation and settling, thewater should not have more than 20 percent ofthe original turbidity. Daily jar tests will helpcheck the optimum chemical dosage required tomeet this standard.COAGULATING CHEMICALSThe type of chemicals that should be usedfor coagulating raw water can be determined byusing the results from jar tests, plant tests, orby using the data shown in table 9-3(A).Theoretically table 9-3(A) is correct; however,these values can be misleading when applied tosome types of raw water. The chemical contentof water may have a considerable influence onthe optimum pH range for the variouscoagulants. For example, coagulation withferrous sulfate is usually best accomplished atrelatively high pH values in the alkaline zone.With soil, colored waters, ferric coagulantmay sometimes be used with considerablesuccess at pH values of 4.0 or less.Because of this wide variation in theoptimum pH range of coagulant (causedby individual characteristics of the rawwater), the coagulant dosage and theoptimum zone for floc formation should bedetermined by jar tests, rather than justrelying on table 9-3(A).Table 9-3(A).Optimum pH Rangesfor Common CoagulantCOAGULANTpHAluminum5.0 to 7.0sulfateFerrous9.5 and abovesulfateChlorinated4.0 to 6.5 andcopperasabove 9.5Ferric4.0 to 6.5 andchlorideabove 9.5Ferric sulfate4.0 to 10.0JAR TESTThe jar test is the most commonmethod of determining proper coagulantdosages. When there is a question as towhich chemical should be used as acoagulant, it is often necessary to runmore than one series of jar tests.Different coagulant chemicals and pHranges should be used to determine whichone produces the most satisfactory resultsat the lowest cost. The step-by-stepprocedures for ajar test are as follows:1. Prepare a standard solution of each9-15